Wiring Sink and Source Digital Inputs on Siemens LOGO! PLCs

Wiring Sink and Source Digital Inputs on Siemens LOGO! PLCs

Siemens LOGO! logic modules are widely deployed for small automation tasks: pump alternation, lighting control, gate interlock, and machine guarding. A common field question is whether a single LOGO! base unit can accept both sinking (NPN) and sourcing (PNP) sensors on the same scan cycle without external interface cards. This reference explains the input architecture of every current LOGO! 8 base module, the three field-proven methods to mix sink and source wiring, and the verification steps that confirm correct commissioning.

Sink vs Source - IEC 61131-2 Definitions

Per AutomationDirect's reference document Sinking and Sourcing Concepts and the IEC 61131-2 standard for programmable controller equipment, every discrete I/O point falls into one of two categories based on conventional current flow into or out of the device terminal.

As clarified in the Honeywell SPS support article, the receiver determines compatibility:

• Sinking Input: the device receiving the signal does not provide the power. It pulls current into its terminal when the sensor is active.
• Sourcing Input: the device receiving the signal does provide the power. It pushes current out of its terminal when the sensor is active.

A sensor and a receiver must always be opposite types. A PNP (sourcing) sensor must feed a sinking (NPN-style) input. A NPN (sinking) sensor must feed a sourcing (PNP-style) input. The LOGO! is internally wired as a sinking input on its I1 through I8 terminals, so it natively accepts PNP (sourcing) sensors.

LOGO! Digital Input Architecture

The current-generation LOGO! 8.3 system manual describes the on-board digital input block as a Type 1 receiver per IEC 61131-2. Each input is internally clamped to a 5.6 V Zener through an optocoupler LED, with a 3 kΩ impedance to 0 V. To register an ON state, the sensor must raise the input terminal to between 15 V and 24 V referenced to the LOGO! power supply common.

This is by design a sink-type input. To trigger the input, the external sensor must source positive voltage onto the Ix terminal. A PNP sensor satisfies this directly. An NPN sensor pulls the line to 0 V, which the LOGO! interprets as OFF. Three workarounds reverse this polarity to convert NPN outputs to a usable +24 V signal.

Compatible LOGO! Modules for Mixed I/O

The 8.3 generation base modules and digital expansion modules listed in the LOGO! 8 system manual differ in input polarity, count, and supply. The table below summarises the units that participate in a mixed source/sink wiring scheme.

Notice the consistency: every digital input on every LOGO! 8 module is a 24 V DC sink-type receiver. There is no LOGO! module that provides true NPN-style or universal polarity inputs. Mixed wiring is therefore achieved at the sensor-side of the wiring, not by selecting a different module.

Method 1 - External 1kΩ Pull-Up Resistor

The simplest and cheapest method is a single pull-up resistor between the LOGO! +24 V supply rail and the input terminal. When the NPN sensor is OFF, the resistor pulls the input to +24 V - the LOGO! reads OFF because no current path exists to 0 V. When the NPN sensor turns ON, it shorts the input to 0 V through its open-collector transistor - but the LOGO! still requires +15 V to register ON.

The previous paragraph is the common mis-wiring that field engineers encounter. A 1 kΩ pull-up will not reverse the logic because the LOGO! is sinking current. A 1 kΩ pull-up is the correct part when the LOGO! input is wired to drive an NPN load, not when the LOGO! is the receiver. The correct configuration for an NPN sensor feeding a LOGO! input is described below.

Important: the topology above produces inverted logic. The LOGO! sees ON when the NPN sensor is OFF, and OFF when the NPN sensor is ON. The ladder program must invert every affected input, e.g. with a NOT block (B001 in FBD) feeding the actual logic.

Resistor sizing notes:

• 1 kΩ @ 24 V → 24 mA standing current. Acceptable for one input; ten parallel sensors means 240 mA of waste heat.
• 2.2 kΩ @ 24 V → 11 mA standing current. Safe margin; works with most open-collector NPN outputs rated ≥ 100 mA sink.
• 4.7 kΩ @ 24 V → 5 mA standing current. Suitable when the sensor specifies minimum trigger current ≤ 1 mA.

For mixed panels with many NPN sensors, a single 1 kΩ pull-up bus is acceptable. The Siemens recommendation in the LOGO! 8 manual is to use ½ W resistors and to keep the common-mode wire short to avoid noise injection. The earlier LOGO! generation manual (LOGO! 8 system manual v8.0) is consistent with this guidance.

Method 2 - LOGO! DM8 24R Expansion Module

The LOGO! DM8 24R (6ED1052-2MD08-0BA1) is a 24 V DC / 24 V AC powered expansion that adds 4 digital inputs (I9 through I12 by default) and 4 relay outputs (Q5 through Q8). Because the relay contacts are isolated, they can be wired back into a separate set of LOGO! base inputs to provide full galvanic isolation. This is the cleanest field method when the NPN sensors and the PNP sensors are powered from different supplies.

Wiring sequence for Method 2:

1. Wire each NPN sensor output to one of the DM8 24R inputs I9-I12.
2. Configure the DM8 24R in the program to use the corresponding relay output as a direct pass-through: Q5 follows I9, Q6 follows I10, etc.
3. Wire each DM8 24R relay contact between +24 V LOGO! supply and the desired base-module input I1-I8.
4. Wire the other side of the relay contact to the LOGO! M (0 V) terminal.
5. Verify in LOGO!Soft Comfort that the input state mirrors the sensor state without inversion.

Method 3 - 8-Channel Relay Interfacing

When the panel already has 8 PNP (sourcing) sensors and 8 NPN (sinking) sensors from two independent machines, an 8-channel interface relay module (Phoenix Contact PLC-OSC or equivalent) is the cleanest retrofit. Each NPN sensor drives a 24 V DC relay coil; the relay output is a dry contact that is wired into a LOGO! input. The LOGO! sees a clean PNP-compatible signal.

This is the method referenced in the field thread: "a set of 8 interfacing relays can be used to condition your sink inputs." The relay module provides:

• Galvanic isolation between the NPN field supply and the LOGO! supply.
• Status LED per channel for commissioning.
• Socket-mounted relays for easy field replacement.
• Pluggable cross-connections on the coil side for jumpered common.

Recommended parts (typical industrial selections):

If a LOGO! expansion slot is available, the Siemens DM16 24R provides the same conditioning with native LOGO!Soft Comfort address mapping. The trade-off is that the LOGO! base recognises the DM16 inputs as I9-I16, leaving I1-I8 free for the PNP sensors directly.

Wiring Procedure for Mixed Source/Sink I/O

The procedure below assumes a panel that must accept 8 sourcing (PNP) and 8 sinking (NPN) sensors on a single LOGO! 24RCE base module. The Method 1 pull-up topology is used because it is the lowest-cost solution and does not consume expansion slots.

Prerequisites

• LOGO! 24RCE base module (6ED1052-1MD08-0BA1) with firmware 8.3.x or later.
• LOGO!Soft Comfort V8.3 (or current release) installed on a Windows PC.
• 8 PNP sensors rated 24 V DC output.
• 8 NPN sensors rated 24 V DC output.
• Eight 1 kΩ ½ W metal-film resistors, ± 5 % tolerance.
• 24 V DC power supply sized for: LOGO! + 8 PNP sensors + 8 NPN sensors + 8 × 24 mA pull-up current ≈ 1 A typical, 2 A peak.

Step-by-Step Procedure

1. De-energise the panel. Verify zero voltage on the 24 V rail with a calibrated multimeter. Lock-out and tag-out the upstream breaker per local arc-flash and electrical safety procedure.
2. Mount the LOGO! on a 35 mm DIN rail inside the enclosure. Leave 35 mm clearance above and below for ventilation; relay output modules dissipate heat through the base.
3. Wire the 24 V DC supply to the LOGO! L1 (+24 V) and M (0 V) terminals. Use 1.5 mm² (16 AWG) stranded copper with ferrules for the power feed.
4. Wire the 8 PNP sensors between LOGO! M (0 V) and inputs I1-I8. The sensor brown wire goes to +24 V; the blue wire goes to LOGO! M; the black output wire goes to the Ix terminal.
5. Install the pull-up resistor network for the NPN sensors on a small terminal block. Bridge +24 V to one side of each resistor; the other side of each resistor becomes a "virtual +24 V source" feeding an Ix terminal. Run 8 jumpers from the resistor block to inputs I9-I16 of a DM16 24R (or to I1-I8 of a base + 1 kΩ pull-up topology if no expansion is fitted).
6. Wire the 8 NPN sensors between the Ix terminal (with the pull-up installed) and the LOGO! M (0 V) terminal. The NPN transistor collector connects to Ix; the emitter connects to M.
7. Connect the LOGO! Ethernet to a PC running LOGO!Soft Comfort for programming and online monitoring.
8. Apply power. The LOGO! power LED illuminates green; the ERROR LED remains off. The display shows "LOGO!" followed by the firmware version.

Logic Configuration in LOGO!Soft Comfort

For the 8 NPN inputs that are inverted, build the program with NOT blocks (B001-B008) at the head of each input leg:

FBD Example - Inverted NPN input:
  I1 (PNP)  ────────► [B001] ──► AND
  I9 (NPN)  ──► NOT [B002] ─────► AND
  Output:               AND ─────► [Q1]  (relay output)

Alternatively, invert in the ladder by inserting a normally-closed contact for the affected NPN inputs. The I-NOT function block is available from the Special / Co library in LOGO!Soft Comfort V8.3.

Verification & Commissioning

Field verification of a mixed source/sink panel follows a 4-step protocol. Each step uses the LOGO!'s built-in online monitor plus a handheld multimeter.

1. Sensor supply check: measure 24 V DC ± 10 % across the brown and blue wires of each sensor. A reading below 21.6 V indicates the supply is undersized.
2. PNP sensor path: with the PNP sensor unblocked, measure the voltage between the Ix terminal and LOGO! M. Expect > 15 V. Below 8 V indicates the sensor is overloaded or the wire is shorted.
3. NPN sensor path: with the NPN sensor unblocked, measure the voltage between the Ix terminal and LOGO! M. Expect < 5 V (the NPN is pulling it to 0 V). With the sensor blocked, expect > 21 V (the pull-up is at work).
4. Logic check: force the input in the online monitor and verify the inverted/not-inverted program branches behave as expected. Use the "I" flag tooltip in LOGO!Soft Comfort to confirm each Ix bit.

For a complete sign-off, generate a LOGO!Soft Comfort PDF report that includes the wiring diagram, the FBD program, the I/O allocation table, and a checklist of the four verification points above.

Troubleshooting Matrix

FAQ